Enzyme synthetic transformations

Biocatalysts usually require mild reaction conditions for an optimal activity (physiologic temperature and pH) and, in general, they show high activity, chemo- and enantioselectivity. Furthermore, when using enzymes, many functional group protections and/or activations can be avoided, allowing shorter synthetic transformations. The use of enzymes is therefore very attractive from an environmental and economic point of view. 

C—H activation has perhaps the highest potential of all enzyme-catalyzed transformations for synthetic applications. At the same time, these transformations are often the most difficult processes to be carried out on a practical scale. Currently, they require whole-cell processes and the outcome is often unpredictable. The discovery of new oxygenases and efficient hosts for protein expression remain keys to further expanding the synthetic applications of enzymatic hydroxylation. 

In conventional synthetic transformations, enzymes are normally used in aqueous or organic solvent at moderate temperatures to preserve the activity of enzymes. Consequently, some of these reactions require longer reaction times. In view of the newer developments wherein enzymes can be immobilized on solid supports [183], they are amenable to relatively higher temperature reaction with adequate pH control. The application of MW irradiation has been explored with two enzyme systems namely Pseudomonas lipase dispersed in Hyflo Super Cell and commercially available SP 435 Novozym (Candida antarctica lipase grafted on an acrylic resin). 

The synthetic route to the three alkaloids is summarised in Fig. 22, in which generation of the catechol is the crucial step based on enzyme-mediated transformation. 

This new style of synthetic catalysis will of course not replace all normal synthetic methods. For many purposes, the standard methods and rules - e.g. aldehydes are more easily reduced than are ketones - will continue to dominate organic synthesis. However, when we require a synthetic transformation that is not accessible to normal procedures, as in the functionalization of unactivated carbons remote from functional groups, artificial enzymes can play a role. They must compete with natural enzymes, and with designed enzyme mutants, but for practical large-scale industrial synthesis there can be advantages with catalysts that are more rugged than proteins. 

The bifunctional nature and the presence of a stereocenter make a-hydroxyketones (acyloins) amenable to further synthetic transformations. There are two classical chemical syntheses for these a-hydroxyketones the acyloin condensation and the benzoin condensation. In the acyloin condensation a new carbon-carbon bond is formed by a reduction, for instance with sodium. In the benzoin condensation the new carbon-carbon bond is formed with the help of an umpolung, induced by the formation of a cyanohydrin. A number of enzymes catalyze this type of reaction, and as might be expected, the reaction conditions are considerably milder [2-4, 26, 27]. In addition the enzymes such as benzaldehyde lyase (BAL) catalyze the formation of a new carbon-carbon bond enantioselectively. Transketolases (TK)... 

In the 1980 s there was a great increase in the development and use of enzymatic procedures by synthetic chemists.6 Previously regarded more as scientific curiosities of limited scope than of practical utility, biological-chemical transformations are now used regularly by synthetic chemists. The ability to induce optical activity in molecules where none existed before is the most useful property of these chiral catalysts. Hydrolase enzymes are generally preferred over other kinds of enzymes for transformations of this nature because they are more easily handled and do not require cofactors for activity. In cases where enantiotopic differentiation between ester functions is desired, prochiral meso diesters are more efficient substrates than racemic esters. In the former case it is possible for all starting material to be converted into a single enantiomer, whereas in the latter example only enzymatic resolution is possible. 

Unless specified otherwise, all reductions included in this chapter gave good yields of >90% enantiomeric excess (ee) products. Not all products of enzyme-catalyzed reactions meet the minimum % ee levels normally required for asymmetric synthetic applications. However, protocols exist for improving ee s of imperfectly specific enzyme-mediated transformations. 

Epoxide hydrolysis is a valuable synthetic transformation. A number of epoxide hydrolase enzymes are known [37]. One of the most interesting and original transformations described using catalytic antibodies is the intramolecular cyclization of racemic hydroxyepoxide 17 (Scheme 7). This compound normally yields tetrahydrofuran 19a following Baldwin s rule. By contrast a single enantiomer of the disfavored product tetrahydropyran 20 a is obtained using a catalytic antibody raised against A-oxide hapten 16 [38]. The same antibody also catalyzes cyclization of hydroxyepoxide 18 to yield optically enriched oxepane... 

In either total synthesis or semisynthesis processing, sometimes a desired synthetic transformation is best done by an enzyme. Such synthesis step, whether using a preparation of the enzyme or the host microorganism, will be considered a chemical synthesis step (a biotransformation or a biocatalytic step) and not a fermentation for biosynthesis. 

Proteases such as a-chymotrypsin, papain, and subtilisin are also useful for regioselective hydrolytic transformations (Scheme 2.40). For example, while regio-selective hydrolysis of a dehydroglutamate diester at the 1-position can be achieved using a-chymotrypsin, the 5-ester is attacked by the protease papain [260]. It is noteworthy that papain is one of the few enzymes used for organic synthetic transformations which originate from plant sources (papaya). Other related protease preparations are derived from fig (ficin) and pineapple stem (bromelain) [261]. 

The reactions invoived in the biodegradation of synthetic biodegradabie poiymers are the same as for naturai poiymers, i.e., enzyme-cataiysed transformations occurring in aqueous media. 

Driven by the major advances in enzyme discoveiy and optimization, for many process chemistry groups, biocatalysis is quickly becoming a synthetic technique of first choice for certain transformations. It is becoming apparent that biocatalysis is optimally employed when opportunities for enzyme-catalysed transformations are considered at the design stage of synthesis, rather than retro-fitted to solve problems with classical chemical transformations... 

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